1. Field
Two part cyanoacrylate/free radically curable adhesive systems are provided.
2. Brief Discussion of Related Technology
Curable compositions such as cyanoacrylate adhesives are well recognized for their excellent ability to rapidly bond a wide range of substrates, generally in a number of minutes and depending on the particular substrate, often in a number of seconds.
Polymerization of cyanoacrylates is initiated by nucleophiles found under normal atmospheric conditions on most surfaces. The initiation by surface chemistry means that sufficient initiating species are available when two surfaces are in close contact with a small layer of cyanoacrylate between the two surfaces. Under these conditions a strong bond is obtained in a short period of time. Thus, in essence the cyanoacrylate often functions as an instant adhesive.
Cyanoacrylate adhesive performance, particularly durability, oftentimes becomes suspect when exposed to elevated temperature conditions and/or high relative humidity conditions. To combat these application-dependent shortcomings, a host of additives have been identified for inclusion in cyanoacrylate adhesive formulations. Improvements would still be seen as beneficial.
A variety of additives and fillers have been added to cyanoacrylate compositions to modify physical properties.
For instance, U.S. Pat. No. 3,183,217 to Serniuk et al. discloses free radical polymerization of a methacrylic acid or methyl methacrylate monomer with a non-polar or mildly polar olefin where the monomer is complexed with a Friedel-Crafts halide.
U.S. Pat. No. 3,963,772 to Takeshita discloses liquid telomers of alkylene and acrylic monomers which result in short chain alternating copolymers substantially terminated at one end of the polymer chains with the more reactive alkylene units. The liquid telomers are useful in making elastomeric polymers for high molecular weight rubbers which permit the ready incorporation of fillers, additives, and the like, due to its liquid phase.
U.S. Pat. No. 4,440,910 to O'Connor is directed to cyanoacrylate compositions having improved toughness, achieved through the addition of elastomers, i.e., acrylic rubbers. These rubbers are either (i) homopolymers of alkyl esters of acrylic acid; (ii) copolymers of another polymerizable monomer, such as lower alkenes, with an alkyl ester of acrylic acid or with an alkoxy ester of acrylic acid; (iii) copolymers of alkyl esters of acrylic acid; (iv) copolymers of alkoxy esters of acrylic acid; and (v) mixtures thereof.
U.S. Pat. No. 4,560,723 to Millet et al. discloses a cyanoacrylate adhesive composition containing a toughening agent comprising a core-shell polymer and a sustainer comprising an organic compound containing one or more unsubstituted or substituted aryl groups. The sustainer is reported to improve retention of toughness after heat aging of cured bonds of the adhesive. The core-shell polymer is treated with an acid wash to remove any polymerization-causing impurities such as salts, soaps or other nucleophilic species left over from the core-shell polymer manufacturing process.
U.S. Pat. No. 5,340,873 to Mitry discloses a cyanoacrylate adhesive composition having improved toughness by including an effective toughening amount of a polyester polymer derived from a dibasic aliphatic or aromatic carboxylic acid and a glycol.
U.S. Pat. No. 5,994,464 to Ohsawa et al. discloses a cyanoacrylate adhesive composition containing a cyanoacrylate monomer, an elastomer miscible or compatible with the cyanoacrylate monomer, and a core-shell polymer being compatible, but not miscible, with the cyanoacrylate monomer.
U.S. Pat. No. 6,833,196 to Wojciak discloses a method of enhancing the toughness of a cyanoacrylate composition between steel and EPDM rubber substrates. The disclosed method is defined by the steps of: providing a cyanoacrylate component; and providing a toughening agent comprising methyl methacrylic monomer and at least one of butyl acrylic monomer and isobornyl acrylic monomer, whereby the acrylic monomer toughening agent enhances the toughness of the cyanoacrylate composition such that whereupon cure, the cyanoacrylate composition has an average tensile shear strength of over about 4400 psi after 72 hours at room temperature cure and 2 hours post cure at 121° C.
Reactive acrylic adhesives that cure by free radical polymerization of (meth)acrylic esters (i.e., acrylates) are known, but suffer from certain drawbacks. Commercially important acrylic adhesives tend to have an offensive odor, particularly those that are made from methyl methacrylate. Methyl methacrylate-based acrylic adhesives also have low flash points (approximately 59° F.). Low flash points are particularly an issue during storage and transportation of the adhesives. If the flash point is 141° F. or lower, the U.S. Department of Transportation classifies the product as “Flammable” and requires marking and special storage and transportation conditions.
U.S. Pat. No. 6,562,181 intended to provide a solution to the problem addressed in the preceding paragraph by describing an adhesive composition comprising: (a) a trifunctional olefinic first monomer comprising an olefinic group that has at least three functional groups each bonded directly to the unsaturated carbon atoms of said olefinic group; (b) an olefinic second monomer that is copolymerizable with the first monomer; (c) a redox initiator system, and (d) a reactive diluent, where the composition is a liquid at room temperature is 100% reactive and substantially free of volatile organic solvent, and is curable at room temperature.
Notwithstanding the state of the art, it would be desirable to provide an adhesive system having both the features of an instant adhesive, such as in terms of the fast fixture times and ability to bond a wide range of substrates such as metals and plastics observed with cyanoacrylates, together with the improved bond strength over a greater variety and/or selection of substrates seen with (meth)acrylate compositions. And it would be desirable to provide a two part reactive adhesive with reduced odor and flammability that could be mixed at a 1:1 volume ratio without comprising shelf life stability or adhesive performance.